Re: [PATCH] tracing: Fix synth event printk format for str fields

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 19 Mar 2025 14:34:19 +0000
Douglas Raillard <douglas.raillard@arm.com> wrote:

> Hi,
> 
> On 19-03-2025 12:18, Masami Hiramatsu (Google) wrote:
> > Hi,
> > 
> > On Tue, 18 Mar 2025 18:09:38 +0000
> > Douglas RAILLARD <douglas.raillard@arm.com> wrote:
> > 
> >> From: Douglas Raillard <douglas.raillard@arm.com>
> >>
> >> The printk format for synth event uses "%.*s" to print string fields,
> >> but then only passes the pointer part as var arg.
> >>
> >> Add the missing precision var arg.
> > 
> > I'm not sure what you want to. Would you mean showing the string length too?
> > But I think actual output(*) shows only string, right?
> > 
> > (*) the output data which can be read from `trace` file.
> 
> The "%.*s" printf/printk format specifier has 2 components:
> 1. the "*" part that in printf documentation called the "precision"
> 2. the "s" part that is called the "specifier"
> 
> For a "s" specifier, the precision is interpreted as the number of chars to display from that string.
> In any case, using "*" as precision means the value is passed dynamically, as a parameter to printf/printk,
> before the actual char *. Both those args are consumed by printk to display "%.*s", the precision is never
> displayed on its own.
> 
> In the current state, synthetic event use "%.*s" for string fields, but then only a single `__get_str(field)`
> is passed to printk, so it's missing the precision arg. Both trace-cmd and our parser report an error because of that.
> 
> The alternative would be to just use "%s" instead of "%.*s", but I assume whoever wrote the code initially had a reason
> to use the precision (maybe to print non-null-terminated strings ?), so I simply fixed the arguments.

Ah, got it. It seems that the first precision parameter is for limiting
the length of the string according to the commit;

https://lore.kernel.org/all/b6bdb34e70d970e8026daa3503db6b8e5cdad524.1601848695.git.zanussi@kernel.org/T/#u

So, I think it should always print the STR_VAR_LEN_MAX value.
Steve, can you check it?

Thank you,

> 
> > 
> > Thank you,
> > 
> >>
> >> Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>
> >> ---
> >>   kernel/trace/trace_events_synth.c | 3 ++-
> >>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
> >> index e3f7d09e5512..274b9b1d9f7d 100644
> >> --- a/kernel/trace/trace_events_synth.c
> >> +++ b/kernel/trace/trace_events_synth.c
> >> @@ -620,7 +620,8 @@ static int __set_synth_event_print_fmt(struct synth_event *event,
> >>   		if (event->fields[i]->is_string &&
> >>   		    event->fields[i]->is_dynamic)
> >>   			pos += snprintf(buf + pos, LEN_OR_ZERO,
> >> -				", __get_str(%s)", event->fields[i]->name);
> >> +				", (int)__get_dynamic_array_len(%s), __get_str(%s)",
> >> +				event->fields[i]->name, event->fields[i]->name);
> >>   		else if (event->fields[i]->is_stack)
> >>   			pos += snprintf(buf + pos, LEN_OR_ZERO,
> >>   				", __get_stacktrace(%s)", event->fields[i]->name);
> >> -- 
> >> 2.43.0
> >>
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>